The use of relatively small hydraulic cylinders is common in many industrial and construction applications. Such applications include Pipe bending, hose crimping, portable hydraulic presses, portable lift jacks, maintenance equipment, etc. Hydraulic fluid is supplied to hydraulic cylinders of such apparatus by relatively portable hydraulic fluid pumps driven by electric motors, small gasoline engines or compressed air motors.
In operation of such hydraulic fluid pumps, hydraulic fluid is forced into a hydraulic cylinder to perform the required work, and then a fluid venting valve will permit return of the hydraulic fluid back to the hydraulic fluid reservoir of the hydraulic fluid pump. In many applications of the prior art hydraulic fluid pumps, venting of hydraulic fluid from the hydraulic cylinder is relatively slow and limits the speed of operation of the hydraulic fluid pump. For example, in the operation of a hose crimping machine of the type commonly used to swage a coupling onto the end of a hydraulic hose, the cycle time for the hydraulic fluid pump supplying hydraulic fluid to a hydraulic cylinder of the hose crimping machine may limit the operator's efficiency in swaging couplings on the ends of the hydraulic hoses.
Prior art hydraulic fluid pumps for use in such applications commonly include an electric motor which drives a low pressure high volume impeller pump and a high pressure low volume piston pump. A ball check valve is provided to selectively permit hydraulic fluid to return to the pump reservoir from a hydraulic cylinder supplied with hydraulic fluid by the pump. The check valve includes a small diameter passage selectively closed by a ball positioned against a valve seat. The ball is held against the valve seat by a relatively large diameter valve piston housed in a valve cylinder provided in the valve. Hydraulic fluid from the impeller pump is supplied to the valve cylinder to cause the valve piston to apply a seating force on the ball.
In normal operation of such prior art pumps, the low pressure impeller pump will maintain a pressure in the cylinder of approximately 120 psi. The high pressure piston pump, on the other hand, may generate a pressure of 10,000 psi which acts to force the ball away from its valve seat. Accordingly, the surface area of the valve piston must be much greater than the cross sectional area of the valve seat if the 120 psi pressure in the valve cylinder is to be effective to hold the ball against the valve seat. In practice, the discharge or vent passage and the valve seat of the valve normally have a diameter as small as 0.10 inches. Due to the small diameter of the vent passage, when the hydraulic cylinder is vented, fluid flow through the vent passage is restricted.